1. Field of the Invention
This invention relates to a process and apparatus for heating a liquid, including water, by using combined direct and indirect heat transfer between hydrocarbon fuel combustion products and the liquid being heated. The process and apparatus of this invention for heating water provide high thermal efficiencies at liquids, including water, temperatures up to about 210.degree. F., ultra-low pollutant air emissions, as well as hot water with very low levels of contaminants.
2. Description of the Prior Art
Processes and apparatuses for heating water in which flue gases from an indirect heating section pass through a direct contact section of a water heater into which water to be heated is introduced and in which heated water from the direct contact section flows into an indirect heating section, where the indirect heating section comprises the combustion apparatus for heating of water in the indirect heating section, are taught by the prior art. One known device produces water heated to a temperature of up to about 203.degree. F.; however, the heated water contains significant amounts of gaseous and liquid contaminants and the device emits high levels of air pollutant emissions.
Ball et al., U.S. Pat. No. 4,658,803, teaches a compact, gas-fired water heater in which water is indirectly heated by a gas burner-fired immersion tube and directly heated by mutual contact between the hot flue gases from the immersion tube outlet and feed water droplets introduced into a direct contact section disposed above the immersion tube as they pass each other in a counter flow arrangement through apertures in a plurality of plates and through a packed bed of grated solid particles.
Similarly, Baker et al., U.S. Pat. No. 4,530,347, teaches a compact gas-fired water heater in which water is indirectly heated by a gas burner-fired immersion tube and directly heated by contact between the hot flue gases from the immersion tube outlet and cold feed water droplets introduced into a direct contact section disposed above the indirect contact section as they pass each other in contraflow through apertures in a plurality of plates.
Wood, U.S. Pat. No. 4,275,708, teaches a direct contact, water heating column furnace for producing heated water having a body and a grid dividing the interior of the body into upper and lower compartments. The upper compartment contains heat absorbing material into which cold water is introduced. The lower compartment forms a combustion chamber and reservoir for storage of hot water heated in the apparatus. The combustion products from the combustion chamber rise through the heat absorbing material to heat the cold water which, in turn, flows around the combustion chamber into the lower compartment.
C. A. Dunham et al., U.S. Pat. No. 2,169,683, teaches an apparatus for producing a heating medium consisting of a highly saturated mixture of steam and hot products of combustion for use in a heating system, the apparatus consisting of a closed housing which is confined in a combustion chamber surrounded at the sides and top by a body of water so that all of the heat not retained by the products of combustion is absorbed by the water. A mixing chamber is disposed above the combustion chamber into which water is sprayed to provide contact with the combustion gases from the combustion chamber, producing a highly saturated mixture. Water condensing therefrom is returned to the water surrounding the combustion chamber.
Devices for heating water employing direct contact heat transfer only and conventional combustion processes are also known. Such designs generally provide thermal efficiencies in excess of 95%, but only heat the water to about 160.degree. F.
Direct contact water heating is also taught by G. C. De Lara et al., U.S. Pat. No. 3,615,079, which discloses a heat exchanger in which a gas and liquid heat carrier are brought into direct contact by the bubbling of the gas in the liquid heat carrier to effect an exchange of heat therebetween and utilizing the bubbling action of the gas in the liquid heat carrier to effect circulation of the liquid heat carrier.
Miyahara, U.S. Pat. No. 3,826,240, teaches a direct contact water heater comprising a body and lattice-like partition plate dividing the interior of the body into an upper heat absorbing chamber and a lower combustion chamber. Combustion products from the lower combustion chamber flow upward into the heat absorbing chamber into which cold water is supplied. A plurality of heat absorbing members are disposed in the heat absorbing chamber and the cold water is supplied onto the heat absorbing members.
Bougard, U.S. Pat. No. 3,648,682, teaches a direct contact water heater in which the liquid to be heated is introduced into the top of a column and distributed in a downward flow through the column. A combustion chamber closed on the top and sides thereof, but open on the bottom, is disposed within the column. Combustion gases produced in the combustion chamber flow through the open bottom of the chamber and upwardly through an annular space, whereby the gases come in contact with the downward flowing liquid to which heat from the combustion gases is transferred.
Lockett et al., U.S. Pat. No. 5,086,731, teaches a gas-fired, direct contact water heater in which the heated water from a lower part of the heater is removed through an outlet conduit. To replace the removed water, water to be heated is sprayed into an upper part of the heater above a heat transfer means through which passes the downward flowing water and upward flowing products of combustion from a combustion chamber disposed in the lower part of the water heater, resulting in heating of the downward flowing water.
Kobayashi et al., U.S. Pat. No. 4,765,280, teaches a direct contact water heater in which the water introduced into the top of the heater flows down along the surrounding sidewalls of a high temperature gas feed chamber in the form of a water film without coming down into the gas feed chamber.
Fullemann, U.S. Pat. No. 4,686,940, teaches an indirect contact device for heating a fluid and cleaning a waste gas comprising a container into which waste gas and an atomized liquid are introduced. A heat exchanger is disposed in a chamber for transferring heat between the waste gas in the container and a liquid to be heated.
As previously indicated, the direct contact of water with combustion products for heating the water in accordance with known water heating devices results in the transfer of substantial gaseous contaminants from the combustion products to the water during the operation of such devices. Thus, water produced by such devices may contain high levels of undesirable gaseous and liquid contaminants. Additionally, pollutants which are not absorbed by the water are exhausted together with other combustion products, thereby emitting high levels of pollutants into the atmosphere. Furthermore, although devices which can achieve high thermal efficiencies at low water temperatures are taught by the prior art, we are unaware of devices which can produce high temperature water, approximately 210.degree. F., at high thermal efficiencies, that is, greater than about 95%.